The broad long-term objective of the proposed research is to further our understanding of the fidelity mechanisms utilized by Pol beta. DNA Polymerase beta (Pol beta) is an enzyme that is known to function in base excision repair (BER) and meiosis. The BER system is responsible for the repair of at least 10,000 lesions per cell per day. Because Pol ? inserts an incorrect nucleotide once for every 10,000 bases it copies, Pol beta has the opportunity to commit 1 error per cell per day during the filling of small gaps resulting from the excision of lesions. These mutations have the potential to result in human disease, including cancer. We will take a combined genetic, biochemical, and structural approach to understand how Pol beta chooses the correct nucleotide substrate for incorporation into DNA. The Specific Aims are: (1) To test the hypothesis that distinct structural sub-domains of Pol beta that we identified during the last funding period are critical for fidelity; (2) To determine if structural sub-domains or amino acid residues of Pol beta other than the ones we have identified previously are critical for fidelity and to understand how they contribute Ito this process; (3) To test the hypothesis that alteration of distinct structural subdomains within Pol beta results in a mutator phenotype in mouse cells. The results of the proposed mechanistic studies of Pol beta will further our understanding of the molecular basis of mutation and have the potential to contribute directly to our understanding of the etiology of significant human diseases, including cancer. Our approach includes identifying and characterizing DNA Polymerase beta mutants using a genetic screen we developed, and analyzing these mutants biochemically and structurally. We also will characterize the phenotypes of Polymerase beta mutants in vivo.